Carbon fiber-reinforced plastics have far superior strength, rigidity and dimensional stability compared with non-reinforced plastics, and therefore, carbon fiber-reinforced resins are widely utilized in various fields such as the fields of business equipment and automobiles. The demand for carbon fibers is increasing every year, and there has been a shift in the demands from premium applications such as aircraft and sporting goods to general industrial applications related to construction, civil engineering and energy industries. Therefore, the requirements for carbon fibers are also strict, and performance improvement as well as cost reduction has been a significant challenge to be addressed. For this reason, carbon fiber bundles having a larger number of filaments and having a higher total fiber fineness have been supplied in recent years, for the purpose of cost reduction.
In general, short carbon fiber-reinforced thermoplastics (hereinafter, described as “CFRTP”) that use thermoplastic resins as matrix resins can be produced with high productivity because they can be processed by injection molding, and the short carbon fiber-reinforced thermoplastics have excellent mechanical characteristics, tribological characteristics, electric characteristics, dimensional stability and the like compared with conventional unreinforced thermoplastics or short glass fiber-reinforced thermoplastics. Therefore, attention has been paid to short carbon fiber-reinforced thermoplastics as a high performance engineering material, and the demand for them is rapidly increasing.
Conventionally, in order to obtain this CFRTP, a method of supplying chopped carbon fiber bundles obtained by cutting into 3 to 10 mm in length carbon fiber bundles bundled with a sizing agent, or milled fibers of carbon fibers that have been pulverized to 1 mm or less, to an extruder together with pellets or a powder of a thermoplastic; pelletizing by melt-kneading; and then molding with an injection molding machine or an extrusion molding machine; is employed.
For the production of chopped carbon fibers, carbon fibers having a number of filaments of about from 1,000 to 30,000 have been hitherto used as raw materials. However, in recent years, in order to cut down the production cost for chopped carbon fibers, measures have been taken to use carbon fiber bundles having a larger number of filaments compared to conventional carbon fiber bundles as raw materials for the production of chopped carbon fibers. That is, there is an increasing need to carry out the production of chopped carbon fibers using carbon fiber bundles having a large number of filaments and a high total fineness.
When a carbon fiber bundle having a large number of filaments and a high total fineness is produced, it is common to handle fiber bundles in a flattened form in order to smoothly carry out the removal of reaction heat at the time of baking.
As a result, in a case in which a chopped carbon fiber bundle is produced using a carbon fiber bundle having a large number of filaments and a high total fineness as a raw material, a chopped carbon fiber bundle having high flatness is produced, for the reason that when the flatness of such a carbon fiber bundle is made higher than that of conventional carbon fiber bundles, the sizing agent can easily penetrate into the interior of the carbon fiber bundle.
On the other hand, as the shape of the carbon fiber bundle becomes flat, there occurs a problem that a chopped carbon fiber bundle having low fluidity or low bundling properties is produced. Also, when the cross-sectional shape approaches a circular shape, the bulk density of the carbon fiber bundle is increased, and it becomes difficult for the sizing liquid to penetrate into the interior of the carbon fiber bundle. Therefore, cohesion of filaments becomes irregular in a bundle. Furthermore, since the shear force exerted in a compounding process is increased, the carbon fiber bundle is easily fibrillated, fiber balls are likely to be generated, and fluidity is decreased. Therefore, when the carbon fiber bundles are transported from a hopper to an extruder in the compounding process, problems such as clogging are likely to occur.
From the past, regarding the method for obtaining chopped carbon fiber bundles, a method of first immersing a carbon fiber bundle in a sizing liquid, subsequently cohering the filaments in the carbon fiber bundle in a drying step, and cutting the carbon fiber bundle with a cutter in a continuous or separate process, has been used commonly. On the other hand, regarding the method of chopping glass fibers, a method of applying a sizing agent to a bundle of melt-spun glass fibers, subsequently cutting the bundle in a wet state, and then drying the chopped bundle, is generally used. According to this method of chopping glass fibers, chopped fibers having high cohesion properties can be easily obtained with a small amount of deposit of the sizing agent, and examples of applying this method to carbon fibers include the inventions of Patent Document 1 and Patent Document 2. However, the number of filaments of a carbon fiber bundle to be chopped in the Patent Documents described above is about 12,000, and the inventions are not intended to treat a carbon fiber bundle having large numbers of filaments and high total fineness. In addition, regarding the chopped glass fibers described above, the number of filaments of a fiber bundle used in the process of applying a sizing agent to the glass fibers is about 4,000, and the process is not intended to treat thick fiber bundles.
As is widely known, various characteristics of CFRTP correlate to the length of the carbon fiber. When milled fibers, which are carbon fibers with a very short length, are used, since the length of the carbon fiber in the molded CFRTP is shortened, the various characteristics are inferior to those of the CFRTP for which chopped carbon fiber is used. Furthermore, although there are occasions in which long fiber pellets, whose length is the same as the cut length of fiber, are used in order to obtain a longer length of the carbon fiber in the CFRTP, difficulty in controlling the fiber orientation in the molded product make the material not very suitable for mass production of CFRTP to be inexpensive. Therefore, generally, chopped carbon fiber bundles are used.
On the other hand, Patent Document 3 discloses a chopped carbon fiber bundle which realizes large-scale packaging to cope with mass production, and can simultaneously satisfy stability in feeding from a hopper to an extruder and dispersibility, and a method for producing the chopped carbon fiber bundle. And it is suggested that the number of carbon fiber filaments that constitute the chopped carbon fiber bundle is of 30,000 to 120,000; the chopped carbon fiber bundle has a ratio (Dmax/Dmin) of the longest diameter (Dmax) and the shortest diameter (Dmin) of the cross-section of 1.0 to 1.8 and is bundled with a sizing agent of 1% to 10% by weight; and the ratio (L/Dmin) of the length of the chopped carbon fiber bundle (L) and the shortest diameter of the chopped carbon fiber bundle (Dmin) is 4 or less.
However, the chopped carbon fiber bundle described in Patent Document 3 have a carbon fiber bundle shape with low flatness, and are easy to cause failure in drying after a sizing agent is applied thereto, resulting in a problem requiring a decrease in the rate of production.
Furthermore, Patent Document 4 suggests a chopped carbon fiber bundle having excellent fluidity and cohesion properties, obtained using an inexpensive carbon fiber bundle of a large number of filaments as a raw material. The patent document suggests a collection of chopped carbon fibers with a sizing agent applied thereto, characterized in that the average weight per unit length in the fiber length direction of a short fiber bundle piece as a constituent unit of the collection is in the range 1.7 to 4 mg/mm, and the coefficient of variation in the distribution of weight per unit length in the fiber length direction is 30% to 60%; and a method for producing the chopped carbon fiber bundle.
However, it is described in Patent Document 4 that the cross-sectional shape of the chop is substantially rectangular, and the length of one side is 1.5 to 6 mm. If the length of one side is shorter than about 3 mm, even if the weight per unit length is 1.7 mg/mm, the cross-section has low flatness. Thus, there is also a problem that drying failure occurs during production, and there is a need to decrease the production rate.
As discussed above, in regard to chopped carbon fiber bundles, it has been difficult to increase fluidity of chopped carbon fiber bundles and to produce large quantities of chopped carbon fiber bundles with high productivity, without deteriorating dispersibility of the carbon fibers or the properties of a molded product.